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Covering the bottom of the engine compartment would also create some other problems.
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i have covered the entire engine compartment of my car and haven't seen any indication of a heat problem,(even with a partial grill block) i did however use thin aluminum for the part that goes over the actual engine bits (oil sump and exhaust) at the front of the front of the tray i have some louvered vents... i'm not sure they do much, but they're there.
one this is that any oil leaks will accumulate on the tray, so some sort of drain is advisable, also you now have to pop the hood to look for leaks rather then spot them on the driveway
i have just have tank to tank results so no accurate data, but subjectively a front and rear undertray made a big difference.
my rear undertray covers mainly the right side as the exhoust sits on the left. as such i only extended the lower edge of the left wheel well (mainly to prevent dirt from getting on top of the undertray) when driving on the highway with this mod for the very first time a very small part of this extention came loose and this caused a loud occilating noise. after popping the little piece back in place the noise was gone on the way back.
also if you have a torsion bar rear suspention you can extend the coroplast tray from the back up to this bar. this could cause it to flex along with the suspention so this scared me at first but in practice it worked flawless. mine is held on to an anti roll bar with zipties and as amateuristic as it may seem it works perfect.
as for wheel skirts i can imagine even half skirts can have a signifficant effect, so if for any reason a full skirt doesn't work, perhaps a "mini skirt" is worth exploring.