My take on improving solenoids

[Hydros]

First would be to add more posts,

The best drill out two extra ports would be to mount the solenoid in a vise that can rotate true. Then, line up the existing ports and rotate the noid 90 degrees., then drill straight through.

1 pic) Just noting there is a small section that need to be shaved off as it touches the S terminal

[Hydros]

The next thing is to use more contact area of the copper washer. Here you would add a washer just inside of the casing to bring the battery terminal closer into the copper washer. You could also concave the inside posts just a little for added contact.

[Hydros]

Next would be to ad another spring just above the plunger or inside of the solenoid windings

[Hydros]

Contact washers, the first in the back sides, the one on the right maybe silver plated.

The second is the contact areas. In the second images, note the area of the washer that is clean, that is the areas the contacts to not touch.

[Hydros]

These are the areas on the contacts that to touch the washers. If you bring the contact inward say 1/16 to 3/32 you would use all the ares of the contact and more area inside of the washer.

Note the hollow contact above the two contacts, see how it looks hollow, unknown why this is, maybe a cheap design, of a way to cool the contacts.

[Hydros]

Noted a larger spring and needs more pressure to compress