This is a modification we've been wanting to do for several months now, and the time was finally right! We had to wait until we were back near our home town so we could put our Happijac benches in storage and give the old desk to Madison (my daughter).
The basis of this modifications very simple. Unbolt the happijac benches and put a desk in their place. The benches were attached with 8 self tapping bolts each. Simply remove said bolts and the benches come right off, leaving the supports to hold the desks. The load ratings for the system is: 600 pounds static; 450 pounds dynamic (in motion), per this page. I'm not sure if that rating is for the whole system (likely) or each side, but even half of those numbers are plenty for the desks themselves as well as the equipment they will hold.
For the desks, I went with plywood so I could stain them the same color as the rest of the “wood” in the RV. However, something like a formica or similar countertop would also be fine so long as you're mindful of the weight. The standard 4′ x 8′ of a single sheet of plywood worked out great because 2′ was exactly the depth I wanted. While I do have a circular saw to cut them myself, both Lowes and Home Depot will cut them for you and it will be a perfect cut. Tip: Choose which side of the plywood you'll want to be the top of the desk and have them cut it with that side UP to avoid having some edge splintering from the saw.
While the plywood might have been fine my itself, I didn't want to take chances with it sagging over time. So, I cut some simple 2×2's to run the length as support. NOTE: make sure your 2×2's are square (not warped) before you buy them. One more trip to home depot for me! Side note: Why on earth are 2×2's really 1.5 x 1.5, and the same with 2×4, 4×4, etc. Never understood that! It's just about as stupid as a measurement system with no common base (12 inches in a foot, 3 feet in a yard, etc). How many Mars landers do we have to crash before we all use the metric system! But, I digress…
To secure the plywood to the 2×2 supports, I simply countersunk #12 1.25″ wood screws through the plywood into the 2×2's. They do need to be countersunk so there will be a deep enough hole to use wood filler. I also used small sections of the 2×2 to support (front to back) under the Happijac support arms, primarily because it looked kind of silly without them, but also as extra support for the front of the desk where my fat hands will be resting.
From there, I used our Dremel tool (awesome for a wide variety of projects) with a router bit to round the edges, then sanded the top and edges with 80, 180, and 400 grit. I then stained (Minwax PolyShades Espresso) the bottom in the morning and top in the afternoon and allowed it to dry inside with the AC on for a day. Note: the proper way to stain would be with 2-3 thin coats, but I opted for the one super thick coat since I didn't want this to drag on for days and I wasn't that concerned about it being showroom perfect.
The last step was to square it up on the Happijac arms and secure it using the same #12 1.25″ wood screws from the bottom (through the happijac bench arms).
After working on the desks for a couple of weeks and making one trip with the monitors strapped down, we are super happy with this modification! Much improved workspaces (particularly for Tara), and much simpler travel mode transition.
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