Yeah none of the cradle dimensions require high precision, and building it is pretty simple. I would build it short enough that the landing light lenses aren't supporting the weight of the wing. You probably won't want to work on the landing light while in the cradle...that would be very inconvenient, but the outboard edge of the wing probably shouldn't be used to support the wing face down in the carpet as there is that narrow edge of aluminum there that you wouldn't want to bend.
I did not sold-bolt my spar end to the cradle. What I did was just slid a couple bolts thru the existing spar holes that went thru the wood. Bolts with the threads removed would be ideal so you don't scrape those holes up. Then I capped the spars with a 2x4 and used clamps to sandwhich the spar between the wood. I countersunk the 2x4 for the bolt heads too. That way I didn't need tools to remove the wing, and I wasn't using hardware to secure it, just quick clamps. Just use care when moving them and DO pin the wings in place somehow. It is possible to have the wings slide out of the cradle if you're not careful and they aren't pinned.
The RV-10 guys have been using such cradles for probably 12-14 years now, so you may even find an RV-10 guy near you who has one they can give you, and you could shorten.