I have noticed the "problem" but have not solution as my rig you get in from the passanger side under cover so you don't get dumped upon
Gutters of some kind are fairly easy to install.... I can design one for you from easily obtained materials in about 10seconds
Use the "U" channel used by a siding contractor to either start or finish the job
Bolt a short section on over the door, Sealing the bolt holes with proper sealant, That's about all there is to it