Firewall for FleetBroadband satellite internet

[MV Alchemy]

I have a FleetBroadband 250 system on my boat that the previous owner installed. I am keen to start using it but know that I need a good firewall between the FB250 and any on-board devices to restrict the types of traffic that can be sent over the satellite link so that costs don't get out of control. Most people seem to use either the Redport Optimizer or the Redbox from Mailasail for this purpose, but both these solutions seem to require a subscription to the vendor's proprietary system (e.g. xgate) which I am keen to avoid.

Can anyone recommend a suitable hardware firewall (preferably with built-in wifi) that I can configure myself and that won't restrict me to a particular vendor's services?

Many thanks,

Tom