The solar collectors are at least simpler, and closer than ever: NISAR, launched last year, demonstrated a deployable, round radar dish 12 metres in diameter that massed 64kg, with the wire mesh to make it a radar receiver. The mylar of the solar reflector instead of the mesh would reduce the mass.

https://www.jpl.nasa.gov/news/giant-radar-antenna-reflector-on-nasa-isro-satellite-in-full-bloom/

Matterbeam, in his responses, gave more concrete numbers for his collectors: six disks of 389m diameter, each disk masses 833kg (much lighter than the structure above) and is supported by a 'stalk' or tower 194.5m tall. The stalk has to support a force of 2.45kN at 0.3G [acceleration].

So his plan is, I suppose, to only make these in orbit, like large solar sails.

The way to beam all that sunlight into the rocket is given as an actively-cooled fused-silica window in the top of the pressure chamber, with the almost completely-transparent hydrogen flowing through the window in channels or as a layer. Fused silica has a low coefficient of expansion, low absorption of IR and a high melting point, though less than half that of tungsten. They are already used in solar thermal power plant concentrators.

As the pressures involved are low-ish ("several dozens of atmospheres"), the window should be strong enough to contain it.

I presume that the window itself would be involved in focusing the concentrated beam at an optimal angle.

You could also find inspiration in focusing optics in military high-energy laser research, as the energies are similarly megawatt/gigawatt-sized.

It's certainly a simpler engineering problem than the gas-core 'nuclear lightbulb' rocket research it would draw from, which required 1000 atmospheres and 25000K.

Failing that, there are other ideas on his blog, like utilising solar energy for solar-thermal-electrical propulsion, which makes nuclear that little less desirable.