I bought this one for mine, it drapes over the hatch easily and you can tuck it in behind the tailgate.
https://www.amazon.com/Coghlans-Single-Rectangular-Mosquito-White/dp/B000KKB2OS?ref_=fsclp_pl_dp_1
Not readily available; but, here is a picture of my lean-to. I took some string and sewed little loops in a few key positions into the tarp - along the top at each end and at middle as well as along the back wall at the pull-outs; then, I hung one of the cheap Couglan's Single Person Bug Box inside after sewing some string onto the bug net that would then tie into the loops I sewed on the tarp. Worked great and was cheap to do.
I think the cheapest you could get with bug coverage would be any cheap tarp you can get and the something like the coghlans mosquito net.
Coghlan's Single Wide Rectangular Mosquito Net, White https://www.amazon.ca/dp/B000KKB2OS/ref=cm_sw_r_awdo_8TA41TGR1CZAA1A2XB2E
You could probably attach something like this to your existing framework, should do the trick.