You should use a dome fixture to help direct the heat and light. It'll also protect yours and your tortoise's eyes from the light that comes out of the base and sides of the bulbs, which can be damaging (it is often higher in UVC).
I have never had any problems using deep domes with ceramic fittings, but I have heard other people swear that MVB bulbs will heat up too much in a normal enclosed dome, and that you need to use a "wire" type fixture or the MVB bulbs will burn out quickly.
Most MVB bulbs are pretty high wattage, so make sure that whatever fitting you use, it is rated above the wattage of the bulb.