Is it a 48" t5 fixture that has a HO ballast? (HO= High Out Put)
The HO is one that can take the standard 54 Watt HO T-5?
That would make the difference what you could use.
If it is an "under cabinet" type light that takes a NO (NO= nominal output) 28-ish watt t-5 bulb... then you will be kind of limited.
Unless you want a zila NO t-5.
But those have had issues and are NOT recommended at all.
Probably in that case you would go with the best NO Full Spectrum 28-ish watt T-5 you can find.
Then you would have to supplement UV B another way by adding an additional light fixture.
The light will be upright in the enclosure facing front to back on the inside of the door, Yes High Output.. I'll be using it for the plants mostly. Is this a good idea?? Here is a picture of the enclosure I built to give you an idea where they will be placed, inside along sides of door in a grooved area designed for these bulbs with screen over them.. I run 2 power sun 100watt full spec. metal valid bulbs in the canopy for Scratchy the past 2 years..