Bolster/guard...up to you I guess!
If the wood's good and well-sealed probably not essential for a fillet knife strength-wise. Guard is personal preference. I'd prefer a slight shaping of the handle--guards get in the way for a lot of food/meat prep.
My limited understanding is that other than looks, they may accomplish the following depending upon design:
Seal off and protect the endgrain of the handle and tang.
A flat plate of the same shape as the end of the handle with a fitted slot for the tang/ricasso does this when glued. If it extends past the width of the handle it becomes a guard. 1/8" brass is OK and easy to work. Aluminum, maybe a little thicker could work too and the color won't contrast with the glue if the fit isn't perfect.. The trick is cutting/fitting the slot. Some blades have a rectangular ricasso which is preferred. Some have no plunge cut and the bevel goes all the way back into the lower part of the tang (Ericsson). Fitting those is a b*tch. You can't get files thin enough for the job. The tang is also radiussed to the ricasso, so careful filing is needed to fit, and probably thicker stock for larger blades. Some like to file that radius on the tang out and get a butt-joint to the wider ricasso instead trying to counter sink a little. I don't like the idea of the stress riser, and the brass is easier to file too.
Spacers are like the above only thicker and can also be made of horn, antler, etc. Added benefit the use a more split resistant material for the part of the handle that gets the most stress. If the wooden handle is thick enough, probably not a big issue for a filet knife. Perhaps a spacer of Corian or other synthetic would work well on the fillet knife. Could be made of metal also, but dunno if spacer would be the right name. Of course, can be combined with a end-plate like above.
My primitive (for USA) tools are a power hand-drill and little files and saws. Fitting a metal spacer would be a bit rough for me. I use brass plate, drill out a row of holes, and start to cut the rest with a cold chisel. Redrill the now smaller holes and chisel again. Pretty soon the chisel will get through and the file works. Lay the thing flat on an improvised anvil and whap it with a hammer if the slot gets a little big. The metal will flow out and decrease the size a bit. Then sand flat. Don't think this would work well if you've got much thicker stock or the metal is steel. If you've got access to a mill, well that's different.
Ferrules and bolsters like HI uses enclose the end of the handle and also strenthen the handle against splitting due to torque. Fabrication is outta my league. Maybe one could cold-hammer brass tube to shape on a mandrel or horn of a tiny anvil. Or hammer sheet cut to right shape over a metal form and solder. Getting the forms is the problem. Also at some point annealing and/or hot hammering will be needed.
Maybe you can get some ideas from the fittings offered at Ragweed Forge.
Let us know how it turns out. I've got a curved piece of peach-tree root that I think I've managed to quick-cure and it's telling me it wants to be a fillet knife handle.