I personally recommend Heavy Bomber Starships, they're quite durable and have good speed. They'll survive detachment from your blob quite nicely. Riot Starships with lasers and occasionally tractor modules and shields are helpful in reducing the chase time. You might not take them down to 25% or 40% speed, but even 10% off will greatly reduce the time your Fighters have to spend in chase. Sometimes if your ships just mosey on over in the direction of the sentinel frigates with out actually targeting them directly, you'll get a lot closer before spooking them...
If you get into bonus ships, Raiders, Vampire Claws (despite the short range tachyon emissions on sentinels), and Vorticular Cutlasses are nice. Spire Teleporting leeches don't do especially fantastic, but are immune to sniping and wouldn't you rather have some sentinels for your own? Sadly, the Leech Starship is too limited in speed to help, but it shares the immunity to sniper shots.
If you're facing a huge number of Sentinel Frigates, particularly in a patrol, setting up a beach-head in the right location is quite helpful. A mk1 fortress will counter zenith bombards, provide repair, general firepower, and is just barely affordable for a rush job and unintended rebuild... Cover it with Missile Turrets, a counter-sniper turret, and occasional fleet support. If you like them as much as I do, a Zenith Spacetime Manipulator or two also makes a big difference.
If all else fails, you could combine plain old Fighters with a MK2 Transport (cloaking and instant unload), or assist Zenith Starships, Maws or other equally slow ships with a Cloaker starship or two.
