Elastic, elastic, elastic
Use some elastic strap or cord to tug gently on the bug net. Strap / band from the sewing supply section of a department store will be fine, as very little strength is needed. If your undershorts aren't held up by their own belt or by the pants over them, then that's all the tug-to-snug you need.
A few inches of that stitched in the right places, will keep your bug net taut even if you shorten the ridge-line.