Take a look at either the Echomax or Tri-Lens radar reflectors, unless you want to spend £450+ on the Sea-Me active antenna.
There is a basemount Echomax 230, but not for the smaller 180 model yet (only mast mount, but you might be able to get something made up).
There is a version with an all-round white that might be useful depending on your existing setup. Note the newer 'extreme' version for 15kts+ use.
The Tri-Lens has 2 sizes you might consider - standard and mini.
Only the large size Echomax and Tri-Lens units actually fulfil the ISO8729 standard but they will probably be too big for you (but one could argue the smaller the boat, the larger the reflector needed).
Either type needs to be mounted clear of obstructions, such as bits of the A-frame, to avoid shadowing.
If you do a Google search for 'radar reflector tests', you'll find various MAIB and QinetiQ performance reports. For a planing boat, you can ignore any negative comments about effectiveness when heeled over - hopefully you won't be! Some of the tests are quite dated and product specs may have changed. Here's the one following the Ouzo incident
As always, there's no substitute for keeping a good watch and knowing what's going on around you.