Late to this topic I see that you are going to make your own, which is fine and I hope fun. I want to throw some experiences out on the table since everything here is in racks, whether or not it was intended to be. Some may be partially repetitious of other posts but no projects are exactly alike so here it is.
Aftermarket shelves as shown in the link suffer from the weakness of having everything supported by the few front holes. These are very thin and of cheapest stamped and bent manufacture. The bolts have to be very tight or they'll ship and the shelf will bend. That Penn Elcom 5U Rack Shelf mentioned is something I'd use only as a last resort or for very light item like a solid state 100W transceiver. I would never put a Viking on something flimsy like that. Heavy items need support front and back.
These may be better:
http://www.budind.com/view/19+inch+Rack+Shelves/Adjustable+Shelf-Open+RackMany racks have rails at the front and another set farther back toward the rear. The best shelves fit that scene, bolting front and back. Angle stock can also be bolted up in that manner to support equipment under the last inch or so of its right and left edges, but for a 100 LB item it's best to use heavy steel or at least 1/4" aluminum. If the gear's feet are not in the way then it will just slide in. A layer of thick nylon tape will ease the sliding-in. Angles are used a lot where a piece of wood is then cut to precisely fit between them as the shelf base. Time honored way for hams. If the rack has no rear rails then brackets can be attached to the rack sides to extend inward and support the rear of a shelf. If angle stock is judged too light then square tubing stock is the next step but the cost and work go up.
Several of the manufacturers of even quite old rack cabinets are still in business. If you can find the name plate or label on the rack, then a phone call can result in a choice of full depth bolt-in shelves made for the rack and they are much stronger than the aftermarket items and always fit. I ordered 15 shelves for a 30 year old three-rack set and the salesperson apologized for a slight lead-time because they would have to make them from the original drawings, set up the machines, etc. and I think they were about $27 each and rated 200 or 300 lbs. Things like the R-390, R-220, NCL-2000, SX-28 sit on them.
Your set is not rackmount so this may not apply, but people say the weight of rigs like R390 should not be supported only by the front panel. (it seemed good enough for FEMA) In some cases the equipment when bolted into the rack does not rest its weight on the shelf but is a fraction of an inch above it. A solution is to make shims of 1/8 and 1/16" steel for the gear to sit on so that its front panel holes align with the rack's mounting holes. Large fender washers work very well, one stack under each corner of the radio. Once this is sorted out, epoxy or silicon glue can be used to stick the washers together and also to the shelf and this is perfect.
If your gear is not rackmount but you want a finished appearance then it is simple to mount a 1U panel across the front of the rack so that when the equipment is pushed forward, its feet or the very bottom edge of its front panel but up against the rear of the panel. Purists may glue a trip of felt on the back of the panel to avoid scratching.
If it's wanted to have the front feet of the gear be the abuttment items, then taller feet can be used so the bottom of the gear extends over the 1U panel with a small clearance. 1U panels can also be cut in half making them 1/2U or 1/3U whatever, but this weakens them and they are more likely to bow forward when the gear is pressed against them.
To strengthen a 1U panel or other panel for use as the front stop, a 3/8x3/8 or 1/2x1/2 inch angle can be bolted to the backside of the panel. It won't bend then.
To hold the gear up against the back of the rack panel, a stout steel angle is used at the rear. It's convenient to make it so that its upright section fits over the grounding stud of the equipment when its lower section is flat on the rack shelf. A hole is drilled in through the angle's lower section and the rack shelf and a bolt with star washer is used to hold it. So the gear is then grounded and held in place and can also be removed by just unscrewing the nut from the grounding stud and removing the 1U panel and sliding it out.
Empty space on either side of the gear can be filled by cutting an inexpensive rack panel vertically so it fits up close to the gear on either side.
There are some pictures of how to mount non-rackmount equipment here. I have done this many times but with an eye toward function rather than beauty:
http://bunkerofdoom.com/rackmount1/index.htmlalso, a way to easily mount really heavy rackmount items without help.
http://bunkerofdoom.com/kd5oei/psurack1/index.html