Well, this is not going to be cheap, but Larsen made a great rectangular base mount. With that Motorola mount, there are other manufacturers. Does that antenna have that microchoke section for 800 mhz? I would almost kill to get my hands on one of those little fellars.
If there are ham radio operators in your area, they might know of a store in your area to obtain one. Sometimes, the base units come with BNC, some don't.
Here is an example of one I found. I believe this will work. It has been some time since I owned one.
Larsen NMOTLP Trunk mount, NMO, UHF/M | TESSCO
Of course, I encourage you to weigh all options for people who want to suggest other models. Hope this works for you.
DWW