Almost any decent splitter may be used outdoors without such an enclosure as long as you use weather-resistant cable connections. A properly installed compression fitting, for example, should last many years.
A weatherproof electrical box with watertight connectors is not cheap but very versatile for mounting components on a mast. A way to attach it could probably be found at a hardware store.. I believe Channel Master had an outdoor combiner splitter for $35. It wasn't water tight either. A real rip-off.
I agree with
; most splitters/combiners are already sealed against moisture. I use Belden Snap n' Seal F connectors, and have never had issues with moisture ingress. Condensation more easily evaporates off components exposed to open air.
On the exterior wall where the antenna coax enters the house and my arrestor is installed, I do have a box for aesthetic reasons only.