There are times when plain old Common Sense has to come into play, and this is one of them - and you are on the right track. The idea of the plastic cloche is to create heat and humidity - but if you've already got both in abundance, then why bother with the cloche?

The cloche acts as a hot-house. Hot houses are usually very large and they have a LOT of space to ensure that plants don't touch the hot sides and tops. If you create a miniature hot-house with a plastic cloche, it needs to be several times larger than the pot under it. Yes? And of course, it helps to keep the temperature inside even.

As long as you don't get large fluctuations in the temperature during a 24-hour period, your cuttings should strike well.

Keep in mind that hot-houses exist even in the hot and steamy tropics. In this situation, they enable EVEN and CONTROLLED amount of heat, water and air-circulation, as required - ie not dependant on outside conditions. Or to put it another way - a hot-house is an attempt at providing plants with their preferred PERFECT conditions.

Live to fish, I'm in Houston too (and I live to fish, too). What kind of lavender do you have? I am growing Godwin Creek and Spanish and another that I don't know the name of. I've grown all from cuttings, and didn't do any of that stuff, I just put the cuttings into pots with dirt and left them alone except for water, for several months. Keep them out of the afternoon sun, dappled shade would be best for now.

As for large temp fluctuations - 98 daytime to 75 nights, not much fluctuation there!

These are Spanish lavender. I did pretty much that. Let us see if it survives. Maybe not because the cuttings have started to turn grey already.

Have you watered your cuttings? Don't allow the soil to dry out - these are babies, and compared with adults, require much more attention. Just make sure the excess water drains away freely.