I have used them for several years.
One nice thing about them is they only warm about 10(F) or so above the outside temp in full sun.

They don't provide much light but the palms are not growing anyway,probably similar to having 6" of snow over your plant,
except that you can heat them.

I use 4 c-9 lights with the Trachys,Needles and Sabals and about 8 with the tree ferns.

Tree ferns-

Basically what it looks like now,except this pic is from late December.

Sounds good Jim. Do you have any issues with condensation like spear pull or anything?

Styrofoam covers work well because they have little
holes in the top to let air exchange-even the T.ferns looked good.