That looks lovely. I know what you mean about not wanting to cover it.
...round dowel to attach to square edges?
...filled in a section of the fuselage with sheet balsa with a round hole for the dowel. It strengthened the attachment point as well.
As P. says. Sheet infill, pierce for dowels. Fit hardwood (spruce usually) dowels after covering.
Looking at yours, for the wing I'd infill just aft of frame D with 3/32" or 1/8" (or two bits of 1/16" glued together cross-grained for strength), doesn't have to be the full gap back to frame E and pierce just forward of the wing in the cockpit "coaming". You might want to put a small piece of sheet doubler inside the coaming before piercing it. Don't have the dowels under the wing and _don't_ cross the bands. Run them fore and aft, that way they are more likely to ping off on a hard arrival saving the wing from damage. Take spare bands with you when you go flying. You will *not* find them when they ping off.
As for fitting the stab? I'd glue it on, but use balsa cement rather than cyano, PVA or aliphatic. It'll simplify construction, cut the weight aft of the CG and you won't have the stab & fin "wandering" between flights. If you use balsa cement you can use cellulose thinners or acetone to dissolve the glue if you find that you need to reposition the stab.
Based on my vast experience (grams: hollow laughter) the hook position is for a hand tow launch rather than bungee. You'll need to move it forward for bungee launching if that's your intention. How much? Ah, now. That's a tricky one that I can't give a definitive answer to. I'd fit two, one at frame B, and one at frame C and see how it goes. For how _not_ to do it see upthread.
If you do want to fly it I'd suggest signing up at HPA (
https://www.hippocketaeronautics.com/hpa_forum/index.php) and heading to the tow-line gliders board. There are people there who really know what they're talking about.
When it comes to covering there are several ways of skinning the cat and every modeller has her or his preferred method. There'll probably be only just enough tissue in the kit. Buy some more, it's dirt cheap, so that when you get it wrong first time you can happily strip it off and try again. As Andy will confirm, 9 out of 10 models need at least one component stripped and recovered *at least* once no matter how many models you've built. The good news is that the one you're building doesn't have any/many compound curves which will make life sooo much easier.
ETA.
There's a very active bungee launch mob at Peterborough who will be very glad to help you get it airborne when it's finished.