Effect of splices on 120 Ω twisted pair cable impedance a.k.a correct way to attach stubs to the bus?
"Don't use stubs just loop in and loop out of each module" as Andy Aka said. UTP rigid cable may not be the best solution. Use a shielded twisted pair cable with characteristics impedance 120 ohm. At each device/stub wrap the wires or crimp them, don't solder them. 
The type of splitters you have mentioned are in my opinion too expensive and they just don't add any benefit to your system, except if you are doing hi-tech military stuff.
I have installed many CAN devices in industrial environment, kind of monitoring system. As a base rule, I used a good shielded cable, correct impedance (UNITRONICS BUS CAN) and no stubs. I'd suggest you not to use UTP, as the rigid wire can break when twisting it, also UTP has no shield.

EDIT:

@Conway @Lundin:I agree. I have misunderstood the question about vehicle CAN aka J1939. However, I would like to underline that it would be good if you crimp wires and use some standard vehicle connector where you will join also power wires. As for characteristics impedance it matters the conductor diameter and overall diameter (as well the \$\varepsilon_r\$ of insulating material). I have found some online calculator:
https://www.allaboutcircuits.com/tools/twisted-pair-impedance-calculator/

The \$\varepsilon_r\$ of the insulator has a standard value regarding the material. If you need perfect impedance match, you have just to choose a suitable wire and wrap them to make a twisted pair. Alternatively you can also inspect CAN wires from a car at junkyard.

Example:
http://www.wiringproducts.com/100ft-spool-red-20-gauge-automotive-primary-wire.html

AWG20 = 0.032 in

distance between conductors == overall diameter = 0.085 in

dielectric constant PVC = 3